1. Field of the Invention
This invention relates to an automatic shutter speed control means for a camera, and more particularly to a shutter speed control means in which the shutter speed, which is automatically determined in accordance with an outpout of an exposure measuring circuit, is corrected when the level of scene brightness is below a predetermined level.
2. Description of the Prior Art
It has been known to control the shutter speed by an electric exposure control circuit in which the level of scene brightness is converted to an electric value by use of a photodetector, and the electric value is memorized as a function of the level of the scene brightness, and the shutter speed is controlled in accordance with the memorized value. In such a shutter speed control circuit, it is common to use a silicon photocell as the photodetector, and use an electromagnet to operate a mechanical shutter in accordance with the electric output of the photocell. The photocell is connected to the shutter by a switching circuit which, in turn, is connected to the photocell by a time constant circuit that includes an integrating capacitor. Particularly in the memory type exposure control circuit, it is known to use a logarithmic diode to convert the output current from the photodetector to an electric voltage to be memorized by the memorizing circuit. The logarithmically converted electric voltage is applied to a time constant circuit that includes an integrating capacitor, and an electromagnet, which is connected with the time constant circuit through a switching circuit, is operated by the output of the time constant circuit.
The quality of the electric elements used in the electric shutter control circuit has now been much improved. Particularly, the dielectric losses of the integrating capacitor and the memorizing capacitor have been reduced. In addition, the dark current of a photocell has been reduced to about one hundredth of prior values, and the stray capacity has been reduced to about one tenth of prior values, and the leakage current of a MOS type FET has also been lowered. Accordingly, the functional conversion by a logarithmic diode is now possible, even when the photocurrent is as low as about 10.sup.-.sup.12 to 10.sup.-.sup.13 A if an operational amplifier using these kinds of improved elements is employed. Therefore, it has become possible to control the exposure time for an extremely low level of scene brightness.
However, the progress and improvements in the shutter speed control circuit have brought about various problems as follows.
If the level of scene brightness is measured by a full aperture measuring system, the minimum level of scene brightness that can be measured by the circuit to control the shutter speed is extremely low. Therefore, it often occurs that the exposure time to be effected actually is objectionably long, e.g. several seconds or several tens of seconds, which is sometimes undesirable from the viewpoint of the practical use of the camera. Further, it sometimes happens that the shutter speed is even controlled for a level of scene brightness below the minimum limit of photographing.
The objectionably long exposure time wastes the driving power of the automatic exposure control camera and thus drains the battery too quickly. Furthermore, the shutter may even be released when no light is incident on the camera, for example when the lens cap is on the taking lens of a single lens reflex camera having this kind of exposure control circuit. If the shutter is released when the photodetector receives no light, the shutter is held open and will not be closed until the electric circuit is turned off by some means. The drain on the battery continues until the shutter is closed.
From the viewpoint of saving the power of the camera, it is desirable to provide a camera with an automatic exposure control of the type in which the shutter is automatically closed when the shutter speed is below a predetermined level.